Thanks for the advice I received a couple of weeks ago on the subject of oil-cooled welders. This last weekend I burnt a couple of pounds of 4 mm electrodes, mostly welding fillets on 3 mm and 5 mm steel. I tried both6011 and 6012 "Weldrite" brand electrodes. I tried to get hold of some of the Murex electrodes that Tim recommended but the welding store didn't have them in stock. I used my Cytringan "Bantam" welder, which is very similar to an Oxford. It provides up to 180 A at 50 V OCV and up to 120 A at 80 V OCV.
The advice I received at the welding store was that some small welders might struggle with 4 mm 6011 electrodes, but that an Oxford-type machine stood a good chance of coping. My machine seemed to cope with running 4 mm 6011 electrodes on 5 mm steel with good penetration. It didn't seem to be struggling either. The 6011 electrodes ran very nicely at 120 A and quite acceptably at 110 A (both at 80 V OCV). My welds looked okay. I still haven't quite mastered the art of a perfect restart and a neater crater fill at the end of a weld, but I guess they're just a matter of practice.
If anything my machine seemed less happy running 4 mm 6012 electrodes, which wasn't what I had expected. Both 4 mm 6011 and 6012 give a recommended current of 120 A to 180 A. The identical current recommendations puzzled me a little as I thought that rutile electrodes needed more current than cellulosics. But 6012 only needs an OCV of 50 V, whereas 6011 needs 70 V, so it gives me the option of running at up to 180 A on my welder. I found that I had to run the 6012s at 165 A to180 A. Apparently 6012 has a sodium binder rutile coating whereas 6013 has a potassium binder rutile coating. As far as I could tell 6012 welded very much like 6013. I bought them mainly because they were cheap (£8 a box) and I was curious to try them.
I also tried running 4 mm 6012s at 120 A and 80 V OCV. This was useless. There was nowhere near enough power. Obviously 120 A at 80 V OCV doesn't give the same power output as 180 A at 50 V OCV. Presumably the voltage once welding has started sinks to a similar value whether you're using the 50 V terminal or the 80 V terminal. I wish I could find a good explanation of how an Oxford-style AC welder works. Does anyone know where I might find one? Maybe I'll end up taking my welder apart sometime. I still haven't got round to fixing the broken internal battery charger and this would give me an excuse to look inside.
I wasn't sure that my welder was entirely happy running at 180 A and 50 V OCV. I wonder if it is perhaps suffering from the voltage drop across the cable from our house to the shed? It is perhaps 40 m of 2.5 mm2 cable (not laid by me, I would have laid a bigger cable). The supply is single phase. This raises the question of whether the shed power supply would cope with a bigger welder. Last week I saw a couple of Oxford RT300 300 amp machines advertised in excellent condition at prices between £75 and £149. They sound like good deals, but there's not much point in having a welder that's beyond the capabilities of our power supply. I must say that I like the 4 mm rods. They burn down more slowly, allow better control of the welding and fewer restarts. So in a way I am tempted by having the capability to run 5 mm rods, but it does seem that for my intended projects with 6 mm steel my 180 A "Bantam" set will be adequate. If it copes with 5 mm steel in the single pass without struggling, I can't see it failing to cope with 6 mm.
But I wonder if I should get some kind of fume extractor. I can't use more than a couple of electrodes in the shed, even with the door open, before I have to leave to let the smoke disperse. I could do with a centrifugal fan as used in laboratory fume cupboards. Unfortunately I passed on one a few months ago because I didn't realise I'd need it. It always happens, doesn't it? I feel that it ought to be the kind of thing I should be able to scrounge from a demolition site, because there must be hundreds that get tossed every year, and they're pretty expensive new.
Thanks for the advice. Comments and thoughts are welcome!